Heat-treating apparatus and method

ABSTRACT

Heat-treating apparatus which includes a controlled atmosphere heat-treating furnace whose exit is closed to the external atmosphere, a conveyor for delivering objects to be treated to the furnace entrance, and a hood covering the conveyor and surrounding the furnace entrance, the entrance of the hood being located at a level equal to or below the floor of the heating chamber of the furnace. Controlled atmosphere gas leaving the furnace entrance is channeled into the hood and is burned therein, thus preheating objects on the conveyor and creating a positive pressure inside the furnace.

United States Patent [72] Inventor William R. Keough Bloomfield Hills, Mich. [2!] Appl. No. 869,755 [22] Filed Oct. 27. 1969 [45] Patented Nov. 16, 1971 73] Assignee Multifastener Company Detroit, Mich.

[541 HEAT-TREATING APPARATUS AND METHOD 7 Claims, 3 Drawing Figs.

[52] [1.8. CI 263/6 R, 263/36, 266/4 R [51] Int. Cl F27b 9/14, F27b 13/06 150] Field of Search 263/6, 8. 36: 266/4 R [56] References Cited UNITED STATES PATENTS 793,268 6/1905 Bates et a1. 263/8 1.168.313 1/1916 Kenworthy 263/6 1.429.380 /1922 Van Marle 263/8 1.860.065 5/1932 Willard et a1. 263/8 2.615.701 10/1952 lpsen 263/8 2.701.712 2/1955 Gilbert 263/6 2.842.352 7/1958 Tauber 263/6 X 3.514.084 5/1970 Morin 263/8 Primary Examiner-John .1. Camby Almrney-Cullen. Settle. Sloman & Cantor ABSTRACT: Heat-treating apparatus which includes a controlled atmosphere heat-treating furnace whose exit is closed to the external atmosphere. a conveyor for delivering objects to be treated to the furnace entrance. and a hood covering the conveyor and surrounding the furnace entrance. the entrance of the hood being located at a level equal to or below the floor of the heating chamber of the furnace. Controlled atmosphere gas leaving the furnace entrance is channeled into the hood and is burned therein. thus preheating objects on the conveyor and creating a positive pressure inside the furnace.

PATENTEnNdv 16 I97! 8,620,517

v SHEET 1 [IF 2 INVHN'IY )R.

\ WILLIAM R. KEOUGH.

BY: SETTLE, BATCI-EL DER 8 ATT'YS OLTMAN.

BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to a controlled atmosphere heattreating furnace and associated apparatus of the type having a chute located at the back of the heat treating chamber of the furnace which connects the heating chamber with a liquid quench bath. After the heat-treating step is completed, the objects being treated are dumped through the chute into the quench bath.

A controlled atmosphere heat-treating furnace is conventionally used in the heat treatment of metal objects when it is desired to prevent decarbonization of and/or carburize the objects while they are being heat treated. If oxygen is present during heat treatment it will react with carbon on the surface of the objects being treated, thus effecting decarbonization of the objects.

- 2. The Prior Art In the prior art, a heat-treating furnace of the type preferred to above has the objects to be treated delivered to the entrance of the furnace on a conveyor. The objects are then transferred by gravity feed through the entrance of the furnace to a conveyor located inside the furnace. The entrance through which the objects are fed is on a plane above the conveyor in the furnace.

A nonoxidizing ,gas is normally used in a controlled atmosphere furnace. The nonoxidizing gas which is introduced into the interior of the furnace can only exit through the entrance of the furnace, since the exit is sealed off by the quench bath.

As the hot nonoxidizing gas exits from the furnace entrance, it comes into contact with oxygen, with combustion resulting. The combustion feeds upon itself, drawing additional nonoxidizing gas out of the furnace at a rapid rate and creating a negative pressure or slight vacuum inside the furnace. The negative pressure draws air into the heating chamber of the furnace from the outside through cracks in the furnace walls. This contaminates the objects being treated as the oxygen in the air decarbonizes them. Since the carbon content of the objects is often of extreme importance, it is extremely desirable to prevent such decarbonization.

It has now been discovered that decarbonization of the objects being treated as a result of air being drawn in through cracks in the furnace walls can be prevented by creating a positive pressure in the furnace. By positive pressure is meant a pressure greater than atmospheric pressure. This is accomplished by placing a hood over the conveyor delivering objects to be treated to the furnace entrance. The hood is attached to and surrounds the entrance of the furnace, and is situated so that the entrance thereof is at a level the same as or lower than the lowest level, or floor, in the heating chamber of the furnace.

This creates a back or positive pressure in the furnace; thus atmospheric air is not drawn into the furnace through cracks in the walls and decarbonization is prevented.

Since the nonoxidizing controlled atmosphere gas is not drawn through the furnace at a rapid rate, as a result of combustion at the entrance. better control of the nonoxidizing gas distribution is achieved. Also, the objects being treated can be carburized by selectively adding carbon containing gases.

It is an object of this invention to provide a controlled atmosphere furnace and associated apparatus, in which a hood surrounds the conveyor delivering material to be treated to the furnace and is attached to the front end of the furnace, confining gases leaving the furnace, causing them to exit at the entrance of the hood which is located at a level equal to or below the lowest level of the heating chamber of the furnace.

Another object of this invention is to provide a method of creating a positive pressure in a furnace heating chamber.

Other objects of this invention will appear in the following description and appended claims, reference being had to the accompanying drawings forming a part of this specification wherein like reference characters designate corresponding parts in the several views.

ON THE DRAWINGS FIG. 1 is a side elevational view, partly in section, of the heat-treating apparatus of the invention;

FIG. 2 is a transverse cross-sectional view through line 2-2 of FIG. 1; and

FIG. 3 is a transverse cross-sectional view through line 3-3 of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring first to FIG. 1, there is shown a heat treatment furnace 10 of more or less conventional construction which takes the form of a thermally insulated tunnellike heating chamber 12, through which objects to be heated are conveyed on the upper run of an endless conveyor belt 14. The belt 14 is trained around a drive roller 16 and an opposite end roller 18, and is driven by a motor (not shown) in a direction such that the upper run of the belt moves from the left to right as viewed in FIG. 1. Heat is imparted to the heating chamber 12 by means of a plurality of burner tubes 20 connected to burner pots (not shown). The furnace 10 has an inlet opening or entrance 22, and a chute 24 surrounding the opening 22 depending toward the forward end of the upper run of the conveyor belt 14.

A gas inlet tube 26 is mounted in the top of the furnace 10, opening into the heating chamber 12. A fan 28 driven by a motor 30 is also mounted in the top of the furnace 10 to circulate the gas being injected through the gas inlet tube 26.

The controlled atmosphere gas most commonly used is a high carbon potential carburizing gas comprising about l9.9% Co, 38.7% H 58% CH 0.2% CO and the balance nitrogen.

Other carburizing gases can be used. In general these contain a relatively high proportion of carbon monoxide, methane, ethane or propane. To minimize the deposition of free carbon particles, or soot, the gases are conventionally diluted with nitrogen or hydrogen. By suitable adjustment of temperature, and gas composition, the surface carbon content and carbon gradient of the objects to be treated can be varied to meet almost any requirement.

Nonoxidizing gases are also used to prevent decarbonization of the metal objects being treated. Gases which are designed to simply prevent decarbonization usually have a low carbon potential. A suitable gas for this purpose is an endothermic gas which basically comprises about 40% H 20% CO, and 40% N The gas which is supplied to the heating chamber 12 through the tube 26 may also be ammonia for nitriding purposes. In general, any desired gas may be introduced into the heating chamber 12 to provide a controlled atmosphere. and the gas may be any desired temperature.

When the objects reach the right-hand end of the conveyor, as viewed in FIG. 1, they drop freely from the belt 14 downward through an outlet opening 32 in the floor of the heating chamber 12 and through a chute 34 which extends downwardly from the opening 32 into a quench bath tank 36. A quench bath of molten salt 38 is contained within the tank 36. The quench bath tank 36 is conventional. A suitable tank is described in US. Pat. No. 3,442,274.

Objects to be conveyed through the furnace 10 are fed onto the upper run of the belt 14, from an infeed conveyor 40. The conveyor 40 comprises a mesh belt 42 having lift bars 44 attached thereto. The belt 42 is trained around a drive roller 46 and support rollers 48. The conveyor 40 is I shaped and for purposes of description can be divided into a lower horizontal section 50, an intermediate vertical section 52, and an upper horizontal section 54. The objects are fed from the upper horizontal section 50 of the conveyor 40 through the chute 24 which depends from the inlet opening 22 in the furnace 10, into the interior of the heating chamber 12. As the objects are conveyed by the belt 14 through the heating chamber 12, they are exposed to the high temperature maintained in the interior of the heating chamber 12 by the burner tubes 20. The temperature within the heating chamber 12 and the speed of movement of the objects through the heating chamber 12 are cooperatively adjusted to achieve the desired heat exposure of the objects during their transit through the heating chamber 12.

An insulated hood 56 comprising a vertically positioned lower section 58 with an entrance 60 at the lower end thereof and a horizontally situated upper section 62 with an exit 64 is mounted over and around the vertical and upper horizontal portions 52 and 54 of the shaped conveyor 40 delivering material to the furnace inlet 22. The hood 56 is rectangular in crosssection and encloses the conveyor 40, as shown. The conveyor belt 42 is supported by angle irons 66 attached to the sidewalls of the upper section 62 of the hood 56. The upper section 62 of the hood 56 is attached to the furnace l and surrounds the inlet opening 22, and the exit 64 communicates with the inlet opening 22 of the furnace 10. The entrance 60 is at a level equal to or lower than the lowest level of the heating chamber 12.

A fan 68 is optionally mounted in the top of the upper section 62 of the hood 56. The purpose of this is to circulate the gas flowing out of the heating chamber 12 and preheat objects being carried on the conveyor 40 for treatment. A plurality of baffle plates 70 are horizontally mounted underneath the fan 68 on angles 72 attached to the side walls of the upper section 62 of the hood 56. The baffle plates 70 aid in achieving even distribution of the gases flowing out of the furnace 10.

A burner 75 is located in the hood 56, this burner operating at a rate to yield 4 percent combustibles. An air vent 74 is also provided adjacent the burner to aspirate air into the hood 56, the vent 74 having a variable orifice controlled by a heat prover to provide the correct amount of aspirated air to burn the endothermic gas entering the hood 56 from the furnace to yield not more than 4 percent combustibles. Actually, the burner 75 acts as a pilot light to insure burning of the en-' dothermic gas. Otherwise, the mixture of endothermic gas and air would be highly explosive. The resultant gases in the hood are exothermic, but nonoxidizing.

By having the entrance 60 of the hood 56 at a level equal to or lower than the lowest level in the furnace heating chamber 12, a positive pressure in the heating chamber 12 results. This prevents oxygen or the atmosphere from being drawn in through cracks in the furnace wall, and thus prevents any substantial decarbonization of the objects being treated. in addition. the nonoxidizing atmosphere gas injected into the furnace is not swept out as rapidly as before, thus better control of carbonization from the injected gas is achieved. Thus if natural gas is added to the nonoxidizing atmosphere, it will react with the objects being treated, causing them to carburize. The degree of carburization is more easily controlled by virtue of the positive pressure in the furnace.

The degree of back pressure achieved with the use of the hood 56 will vary. depending on how long the hood 56 is, how large the entrance 60 in the hood 56 is, and how far the entrance 60 is below the lowest level in the heating chamber 12. The smaller the entrance 60 and the lower the entrance 60, the greater the back pressure in the heating chamber 12 will be. It has been found a back pressure of about 0.10 pound per square inch in excess of atmospheric pressure is satisfactory.

This invention can be more fully understood by reference to the following example, which is intended to be illustrative, but no limiting.

EXAMPLE A controlled atmosphere heat-treating furnace was constructed having a heating chamber with a continuous conveyor in the chamber for moving objects to be treated therethrough. The heating chamber had a volume of about 500 cubic feet. The conveyor dumped the treated objects into a chute located at the back of the heating chamber. The chute emptied into a molten salt quench bath. A controlled atmosphere was injected into the heating chamber at the rate of about 2,000 cubic feet per hour Objects to be treated were delivered to the entrance of the furnace on a I shaped conveyor. The entrance of the furnace was on a higher level than the conveyor in the heating chamberplt was 3 feet wide and 7 inches in length. Objects delivered to the entrance were gravity fed onto the conveyor in the heating chamber, by means of a chute.

A hood was attached to the entrance of the furnace, and surrounded the vertical and upper horizontal sections of the conveyor delivering objects to be treated to the furnace entrance, for a distance along the conveyor of about 20 feet. The entrance to the hood was located approximately 2 feet below the level of the floor of the heating chamber and was approximately 3 feet wide and 10 inches long. The controlled atmosphere leaving the entrance of the furnace was channeled through the hood, and burned in the hood to a resultant exothermic, nonoxidizing gas. The back pressure in the heating chamber of the furnace had a pressure of about 0.1 p.s.i.. and was effective to prevent the external atmosphere from being drawn into the heating chamber.

1 claim:

1. Heat-treating apparatus comprising: l) a controlled endothermic atmosphere heat-treating furnace having an entrance and an exit, said exit being closed off to the escape of said controlled atmosphere by a quench bath, and furnace conveyor means for carrying objects to be treated through said furnace; (2) conveyor means for delivering objects to be treated through said entrance of said furnace to said furnace conveyor means; (3) a hood attached to said furnace and enclosing said entrance of said furnace and a portion of said conveyor means delivering objects to be treated, said hood having an outlet located on a plane at least as low as the interior floor of the heating chamber of said furnace, and (4) means for combusting the furnace atmosphere in the hood, whereby said endothermic atmosphere leaving said entrance of said furnace is channeled through said hood and combusted therein to preheat objects being carried on said conveyor means, and is ultimately discharged through said outlet of said hood, creating a positive pressure in said furnace.

2. The apparatus of claim I wherein said conveyor means for delivering objects to be treated comprises a lower horizontal section, an intermediate vertical section, and an upper horizontal section; and wherein said hood comprises a lower vertical section surrounding said intermediate vertical section of said conveyor, and an upper horizontal section surrounding said upper horizontal section of said conveyor; and wherein said outlet of said hood is at the lower end of saidintermediate vertical section, the dimensions of said outlet and its location on a plane equal to or lower than the interior floor of the heating chamber of said furnace being sufficient to create a back pressure in said furnace of about 0.1 p.s.i., greater than atmospheric pressure.

3. The apparatus of claim 1 which additionally includes a fan mounted in said hood for circulating gases through said hood.

4. in a nonoxidizing atmospheric heat-treating furnace system which includes a nonoxidizing atmosphere heat treating furnace containing a conveyor for transit of metal articles being treated, a discharge chute at the end of the conveyor, a quench bath at the end of the discharge chute, and a conveyor for delivering articles to be treated to the furnace entrance, wherein the only substantial exit for the nonoxidizing atmosphere is through the entrance of the furnace, the improvement of a hood enclosing said conveyor delivering articles to be treated to said furnace entrance, said hood having an entrance and an exit, said exit being in communication with said entrance of said furnace and said entrance of said hood being located at a level equal to or lower than the lowest level in the heating chamber of said furnace, said hood channeling nonoxidizing atmosphere leaving from said entrance of said furnace over said conveyor delivering articles to be treated and out said entrance of said hood, whereby said articles to be treated are pre-heated, the dimensions and location of said entrance of said hood being such that a positive pressure is built up inside said furnace, thus preventing external atmosphere from being drawn into said furnace and decarbonizing the articles being treated.

5. ln the system of claim 4, the further improvement of a fan mounted in said hood for circulating gases leaving said furnace entrance over said articles being delivered to said furnace entrance on said conveyor.

6. A method of creating a positive pressure inside a controlled endothermic atmosphere heat-treating furnace of the type wherein the controlled atmosphere gases leave the furnace at the entrance to the furnace through which the workpieces are delivered, comprising channeling the gases leaving the entrance of the furnace through an enclosed hood surrounding the conveyor delivering objects to be treated, combusting the channeled gas in the hood, allowing combusted gas to escape from the hood, at a level equal to or lower than the lowest level inside the heating chamber of the furnace, thereby creating a positive pressure inside the furnace and preheating objects on said conveyor prior to their entry into the furnace.

7. A method as defined in claim 6, wherein the gases in said hood contain a maximum of 4 percent combustibles and said gases are exothermic in nature, but nonoxidizing. 

1. Heat-treating apparatus comprising: (1) a controlled endothermic atmosphere heat-treating furnace having an entrance and an exit, said exit being closed off to the escape of said controlled atmosphere by a quench bath, and furnace conveyor means for carrying objects to be treated through said furnace; (2) conveyor means for delivering objects to be treated through said entrance of said furnace to said furnace conveyor means; (3) a hood attached to said furnace and enclosing said entrance of said furnace and a portion of said conveyor means delivering objects to be treated, said hood having an outlet located on a plane at least as low as the interior floor of the heating chamber of said furnace, and (4) means for combusting the furnace atmosphere in the hood, whereby said endothermic atmosphere leaving said entrance of said furnace is channeled through said hood and combusted therein to preheat objects being carried on said conveyor means, and is ultimately discharged through said outlet of said hood, creating a positive pressure in said furnace.
 2. The apparatus of claim 1 wherein said conveyor means for delivering objects to be treated comprises a lower horizontal section, an intermediate vertical section, and an upper horizontal section; and wherein said hood comprises a lower vertical section surrounding said intermediate vertical section of said conveyor, and an upper horizontal section surrounding said upper horizontal section of said conveyor; and wherein said outlet of said hood is at the lower end of said intermediate vertical section, the dimensions of said outlet and its location on a plane equal to or lower than the interior floor of the heating chamber of said furnace being sufficient to create a back pressure in said furnace of about 0.1 p.s.i., greater than atmospheric pressure.
 3. The apparatus of claim 1 which additionally includes a fan mounted in said hood for circulating gases through said hood.
 4. In a nonoxidizing atmosphere heat-treating furnace system which includes a nonoxidizing atmosphere heat treating furnace containing a conveyor for transit of metal articles being treated, a discharge chute at the end of the conveyor, a quench bath at the end of the discharge chute, and a conveyor for delivering articles to be treated to the furnace entrance, wherein the only substantial exit for the nonoxidizing atmosphere is through the entrance of the furnace, the improvement of a hood enclosing said conveyor delivering articles to be treated to said furnace entrance, said hood having an entrance and an exit, said exit being in communication with said entrance of said furnace and said entrance of said hood being located at a level equal to or lower than the lowest level in the heating chamber of said furnace, said hood channeling nonoxidizing atmosphere leaving from said entrance of said furnace over said conveyor delivering articles to be treated and out said entrance of said hood, whereby said articles to be treated are pre-heated, the dimensions and location of said entrance of said hood being such that a positive pressure is built up inside said furnace, thus preventing external atmosphere from being drawn into said furnace and decarbonizing the articles being treated.
 5. In the system of claim 4, the further improvement of a fan mounted in said hood for circulating gases leaving said furnace entrance over said articles being delivered to said furnace entrance on said conveyor.
 6. A method of creating a positive pressure inside a controlled endothermic atmosphere heat-treating furnace of the type wherein the controlled atmosphere gases leave the furnace at the entrance to the furnace through which the workpieces are delivered, comprising channeling the gases leaving the entrance of the furnace through an enclosed hood surrounding the conveyor delivering objects to be treated, combusting the channeled gas in the hood, allowing combusted gas to escape from the hood, at a level equal to or lower than the lowest level inside the heating chamber of the furnace, thereby creating a positive pressure inside the furnace and preheating objects on said conveyor prior to their entry into the furnace.
 7. A method as defined in claim 6, wherein the gases in said hood contain a maximum of 4 percent combustibles and said gases are exothermic in nature, but nonoxidizing. 